The present invention relates to acoustic logging tools for performing acoustic investigations of subsurface geological formations traversed by a borehole. More particularly, the invention relates to the implementation of particle damping in well logging tools to attenuate tool waves.
In the oil and gas industry, subsurface formations are typically probed by well logging tools to determine formation characteristics which can be used to predict or assess the profitability and producibility of subsequent drilling or production operations. In many cases, acoustic logging tools are used to measure formation acoustic properties which may be used to produce images or derive related characteristics for the formations.
Acoustic waves are periodic vibrational disturbances resulting from acoustic energy which propagates through a medium, such as a formation or logging tool. Acoustic waves are typically characterized in terms of their frequency, amplitude, and speed of propagation. Acoustic properties of interest for formations may include compressional (P) wave speed, shear (S) wave speed, and borehole modes, such as tube wave. Additionally, acoustic images may be used to depict borehole wall conditions and other geological features away from the borehole. These acoustic measurements have applications in seismic correlation, petrophysics, rock mechanics and other areas.
Recordings of acoustic properties as functions of depth are known as acoustic logs. Information obtained from acoustic logs may be useful in a variety of applications, including well to well correlation, determining porosity, determining mechanical or elastic parameters of rock to give an indication of lithology, detecting over-pressured formation zones, and enabling the conversion of a seismic time trace to a depth trace based on the measured speed of sound in the formation.
An acoustic logging tool typically includes one or more acoustic sources (i.e., a transmitter) for emitting acoustical energy into subsurface formations and one or more acoustic receivers for receiving acoustic energy. The receivers are typically axially spaced apart from the transmitters to allow the acoustic energy to propagating through the surrounding formation before being received at the receivers.
Transmitters and receivers for acoustic logging tools commonly comprise acoustic transducer elements, such as piezoelectric crystals. In general, an acoustic transducer converts energy between electric and acoustic forms and can be adapted to act as a source or a receiver. Acoustic transducers are typically mounted on the body of the logging tool. It is desired that the minimum amount of energy from the transmitter be transferred to the tool body and the maximum amount of energy be radiated into the borehole and the formation.
Acoustic energy emitted from a logging tool in a borehole may travel along multiple paths to reach the receivers. The part of the acoustic energy that propagates through the formation and fluid in the well is the energy that provides useful information for characterizing the formation. The part of the acoustic energy that propagates through the tool body generally provides no useful information about the formation and often presents a difficulty in measuring acoustic information from the formation.
A common issue for all acoustic tools is the part of the acoustic energy propagating along the tool body, referred to as a xe2x80x9ctool wave.xe2x80x9d Tool wave is undesired because it contains substantially no information about the formation and interferes with the part of the acoustic energy propagating through the formation, referred to as the xe2x80x9cformation wave.xe2x80x9d For many wireline tools, unwanted tool wave is reduced with design features such as slotted sleeves, isolation joints and flexible tool bodies. For logging while drilling (LWD) tools, tool waves are an even more serious challenge because these waves are carried by the prominent and stiff tool body, which is essentially a drill collar.
Various forms of acoustical energy propagating in the borehole can be used for probing different properties of the surrounding formation. For example, a monopole logging tool (wireline or logging while drilling type) uses single or multiple monopole acoustic source(s) as well as receivers which oscillate and detect uniformly in all azimuthal directions in the plane perpendicular to the tool axis.
It is well understood based on theory of wave propagation that a monopole tool can excite and detect P-waves and Stoneley waves in substantially all formations, regardless of formation acoustic speed. In addition, a monopole tool is capable of generating and detecting S-waves in so called xe2x80x9cfastxe2x80x9d formations where the formation shear speed is faster than the sound speed in the borehole fluidxe2x80x94drilling mud. However, part of the energy emitted by the monopole source couples to the tool body and generates tool waves. This tool wave propagates at a speed of about 5000 m/sec for low frequencies in a steel mandrel and typically arrives at the receivers before almost all the desirable signals from the surrounding formation. As a result, this tool wave arrival interferes with the desired formation wave signals, especially the formation P-wave.
For wireline monopole tools, the tool waves are usually delayed and suppressed by techniques such as slotted receiver housings. In logging while drilling monopole tools, which require thicker and stronger tool bodies, suppressing tool waves has proven to be a more difficult issue. One logging while drilling monopole tool operated under the trademark ISONIC by Schlumberger Technology Corporation of Sugar Land, Tex., achieves tool wave attenuation over a selected frequency band with a specially designed periodic array of grooves machined on the collar section between the transmitter and receivers, as described in U.S. Pat. No. 5,852,587 to Kostek et al.
As another example, a wireline dipole tool generates and receives flexural mode waves in a borehole. The term dipole refers to the azimuthal profile cosxcex8 for the transmitter, receivers and the acoustic field associated with the flexural mode. The flexural mode propagation speed asymptotes to the formation shear speed at the low frequencies, and to the mud-formation interface wave speed at high frequencies. Thus S-wave speed of the formation can be derived from the measured flexural mode as discussed in xe2x80x9cAcoustic multipole sources in fluid-filled boreholesxe2x80x9d by Kurkjian and Chang in Geophysics, 51, 148-163 (1986).
To avoid or minimize tool wave effects on the measured borehole flexural mode, wireline dipole tools commonly use acoustically slow (i.e., mechanically flexible) housings for receivers. These tools may also include a form of acoustic isolator or attenuator between the source and receivers to reduce the transmission of tool waves.
Applying the wireline dipole shear technique to LWD tools is difficult. First, LWD tools cannot be made very flexible or acoustically slow, as done for wireline tools, because the tool body of an LWD tool is, in most cases, essentially a drill collar. This provides an easy propagation path for acoustic energy between the acoustic source and the receivers. The tool wave interferes with the borehole flexural wave and makes the measurement much more complicated and difficult, as discussed in xe2x80x9cMandrel effects on the dipole flexural mode in a boreholexe2x80x9d by Hsu and Sinha in Journal of the Acoustical Society of America, 104(4), 2025-2039 (1998) and xe2x80x9cAcoustics of fluid-filled boreholes with pipe: Guided propagation and radiationxe2x80x9d by Rao and Vandiver in Journal of the Acoustical Society of America, 105, 3057-3066 (1999).
Additionally, in some cases acoustical energy reflected from formation or tool discontinuities above and below the acoustic transmitter and receivers, and acoustical energy coupled from the surrounding formation back to the tool may interfere with measurement quality or affect tool durability.
Several approaches for reducing tool waves have been proposed. For example, U.S. Pat. No. 5,510,582 disclosed the idea of attenuation by using a thin layer of viscous fluid in between an inertia mass and a cavity in the tool body. The relative motion between the tool and the inertia mass results in viscous dissipation in the fluid. U.S. Pat. No. 6,082,484 disclosed the concept of employing fluid-filled cavities on the tool body with the cavity resonance frequency designed within the frequency band of interest. U.S. Pat. No. 5,936,913 disclosed removing tool waves by providing a compensating sensor for detecting tool vibrations and for producing signals, which are combined with waveform data.
In a quest for attenuating these interfering tool waves, new techniques may be considered and adapted for use in downhole tool environments. One passive technique that has been used to reduce structural vibration is discussed in Vibration Damping, by Nashif, Jones, and Henderson (Wiley-Interscience, 1985). This technique involves the dissipation of energy through the shearing of a thin layer of viscoelastic material between a structure""s surface and a thin constraining shell and, thus, is known as the xe2x80x9cconstraint layerxe2x80x9d technique. However, the availability of a suitable viscoelastic material for the downhole temperature range is a major challenge.
Another method for structural damping is particle damping. Particle damping is a method for structural damping involving the use of particle-filled enclosures as part of the structure, as disclosed in U.S. Pat. No. 5,365,842 to Panossian. Using particle damping, energy is dissipated through the inelastic collision and friction between particles and the enclosure. This method may be applied to logging tools, such as acoustic logging while drilling and wireline tools to attenuate tool waves along the tool body. Additionally, this method is expected to be effective for a wide frequency band and suitable for downhole environment especially because of its temperature insensitivity.
Therefore, the incorporation of particle damping for downhole tools, and in particular for logging tools, is proposed.
One aspect of the invention is an acoustic logging tool for performing acoustic characterizations of subsurface geological formations. In one embodiment, the tool comprises a generally longitudinally extending tool body adapted for positioning in a borehole. At least one transmitter is mounted on the tool body. At least one receiver is also mounted on the tool body at a location axially displaced from the at least one transmitter. An attenuator is disposed along the tool body. The attenuator comprises at least one cavity having a plurality of particles disposed therein. In some embodiments, a fluid may also be disposed in the at least one cavity.
One or more attenuators comprising one or more cavities with particles disposed therein also may be applied to other downhole tools for reducing tool vibrations to protect tool components or to improve measurement quality.
In another aspect, the invention provides a method for attenuating acoustic energy propagating through a body of an acoustic logging tool. In one embodiment, the method comprises emitting acoustic energy from a first location on the tool; acoustically exciting particles disposed in at least one cavity acoustically coupled to the tool body at a location along an acoustical path through the body from the first location of emitting to a second location of receiving; and receiving attenuated acoustic energy propagated through the body at the second location of receiving.
Other embodiments, aspects, and advantages of the invention will be apparent from the following drawings and description and the appended claims.